The clipboard in mac isn't viewable without an application like Windows Clipboard Viewer. One Option for you might be iClip Light Dashboard Widget. Might be a bit overblown for your needs, but it might just be useful for you if your someone who works a lot with the clipboard. iClip full version is a good piece of software, too.

After a bit of playing around, here's your answer:cmd-shift-3 takes the whole screen, svaes it to the desktop (Doesn't go to the clipboard)cmd-shift-4 lets you select an area on the screen, and saves it.

cmd-ctrl-shift-3 takes the whole screen, and copies it to clipboard.cmd-ctrl-shift-4 allows you to select an area on screen to copy to clipboard.

Seeing as this thread has been revived from the dead, in case anyone's interested in creating multiple clipboards for copying and pasting, check out Clip Doubler. It's freeware from Northern Softworks.
Whenever you copy, Clip Doubler creates a new clipboard. This way you can refer back to previous clipboards without having to resort to opening multiple documents. Cheers!